1. Field of the Invention
The present invention deals generally with the early detection of power failure resultant from the loss of alternating current (AC) power supplied to an electrical system.
2. Background Description
Many electrical and electronic systems using alternating current, AC, power require early notification of the failure of such power so that orderly cessation of operations can be accomplished. For example, in computer systems with volatile memory a timely warning of a primary power outage is often used to initiate the transfer of the contents of volatile memory stores to non-volatile memory stores. The contents can be subsequently retrieved from the nonvolatile stores upon resumption of adequate power to support processing.
Loss of the alternating current power satisfactory to operate electrical and electronic systems can occur in two basic ways: a "brown-out" characterized by reduced AC amplitude, or a "black-out" characterized by a sudden and complete loss of power. Detection of both occurrences as soon as possible is beneficial in order to give the system maximum time to respond to what is normally considered an undesired and unscheduled failure event.
In one previous method of detecting the failure of alternating current power, the peaks of the alternating current power voltage waveform are monitored. Normally the voltage waveform is half-wave rectified, and the half-wave rectified voltage waveform is sensed to determine if the waveform crosses a single voltage threshold. This threshold is of a voltage value substantially equal to the normal maximum voltage magnitude and is normally crossed in each direction twice during each complete cycle. This method is generally satisfactory for detecting a power "brown-out", which is defined as a condition in which the peak of the power voltage waveform fails to reach its full normal voltage excursion. However, in the event of a complete loss of AC power, a peak detection method generally does not provide a power interruption alarm any sooner than one-half cycle after the last occurring peak, irrespective of the point in the cycle at which one half cycle for a 60 Hertz alternating current waveform is 8 milliseconds in duration, the sudden loss of alternating current may not be detected by a peak-detecting AC power failure detector until up to 8 milliseconds after the actual interruption of power. For electronic systems in which processing operations are typically conducted within microseconds, an 8 millisecond interval may be too long to prevent loss or corruption of data. If an earlier detection of the complete interruption of alternating current power could be accomplished, then the electrical or electronic system using such power could be earlier alerted of the power interruption and could be accorded more time in which to effect an orderly cessation of those ongoing operations dependent upon such power.
One previous system directed to the detection of a power failure within a fraction of a cycle to enable the contents of a volatile memory to be converted into a nonvolatile form before loss occurs is described in U.S. Pat. No. 3,937,937 for PRIMARY POWER FAULT DETECTOR to McVey. The circuit of McVey detects both voltage and current in order to calculate within an integrated circuit multiplier device the root mean square (RMS) power as the product of such detected voltage and current. If the power fails, the calculated RMS power will equal zero, providing an indication of the loss of power.
Another previous system for detecting the interruption of power is described in U.S. Pat. No. 4,473,756 for AC UNINTERRUPTIBLE POWER SYSTEM to Bridgen et al. In Bridgen et al., a dual line sensor circuit compares the normal AC power line signal (transformer coupled) with a reference AC signal, and under normal line conditions delivers a logic "one" state signal to a switch control circuit and to an alternate power control circuit. When the input AC power signal and the reference AC signal are not in agreement, then the power is assumed to either have failed or degraded. Upon such a failure or degradation a switch is first opened for disconnection of the failed or degraded power, and then, after a short delay, an alternate power source will be coupled to the electronic system. Such detection, as well as provision of uninterruptible AC power, depends upon the availability of a second, independent, reference AC signal.
Other types of power loss detectors utilize zero crossing detectors. In U.S. Pat. No. 3,955,102 for ZERO CROSSING DETECTION CIRCUIT to Chi, an input signal is applied to both positive and negative threshold circuits which are each connected to respective one-shot circuits. The output pulses of both one-shots are applied to a NOR circuit, thereby responding to the beginning edges but not to the trailing edges of signals. Accordingly, variations due to hysteresis or to varying propagation time in the trailing edges of the signal for which zero crossing is being detected do not affect the detection of the zero crossing.
In U.S. Pat. No. 4,229,669 for TIGHT TOLERANCE ZERO CROSSING DETECTOR CIRCUIT to Smith, a zero crossing detector provides an optically coupled output signal at the zero crossing time. A complex impedance matching circuit is provided to enable tight tolerance detection of zero crossing, which is particularly useful for the recognized advantage of switching loads at or near the zero crossing point of the power supply waveform.
Finally, in U.S. Pat. No. 4,480,200 for ZERO-CROSSING POINT DETECTION CIRCUIT to Tan et al., the upward and downward transitions of an input signal are independently detected with Schmitt trigger circuits which are asymmetrical in different directions with respect to the hysteresis of the waveform. The detection signals from the Schmitt triggers are then combined in an exclusive OR circuit to provide zero crossing level detection having no time lag and substantial immunity to small noise signals.